MEMS devices with integrated IC electronics are being used in many consumer applications. For example, MEMS accelerometers are used in air bag deployment, MEMS gyros provide hand jitter information for image stabilization in digital cameras, MEMS microphones are replacing conventional electret microphones in cell phones, vibrating mirrors enable ultra small projectors for consumer applications. In the earlier MEMS devices, the sensor output was usually provided to the outside world by analog signals. In recent years, consumer applications started to require additional features from the MEMS devices. These additional features include providing digital outputs, digital filtering, and interrupt generation upon detecting certain ranges of sensor data. Even further, many inertial sensors include algorithms or features for detection of complex movements and gestures. These additional features are implemented using digital circuits and the accuracy of these circuits is determined by the accuracy of the clock signal. Especially, for navigation applications, accuracy of the timing is crucial. Applications such as GPS assist and dead reckoning rely on integration of motion sensor output to determine orientation and position. The accuracy of the integration time steps is determined by the accuracy of the clock.
There are two common types of oscillator circuits: relaxation oscillators and harmonic oscillators. In relaxation oscillators, an energy storing device (capacitor or inductor) is charged and discharged by a non-linear circuit component. This cycle is repeated indefinitely creating a periodic signal which is usually a saw tooth wave. FIG. 1 shows an example of a relaxation oscillator circuit. In this circuit, the capacitor CC (103) is charged through the resistor RC (102). The voltage at node A increases as the capacitor charge increases. The Schmitt trigger 101 output stays low if the input is below certain threshold value, LH, or goes to high if the input exceeds another threshold value, HI. The capacitor voltage or the voltage at node A in this circuit controls the output of the buffer. When the voltage exceeds the high threshold voltage of the Schmitt trigger buffer 101, the buffer output becomes logic high closing the switch 104. The capacitor is discharged through the switch 104. When the capacitor voltage drops below the low threshold, Schmitt trigger buffer 101 output goes back to logic low opening the switch 104. At this point, the resistor starts charging the capacitor again. This cycle repeats continuously creating a square wave at the buffer output. The oscillation frequency of the circuit is determined by the RCCC time constant. Small changes in the R or C values directly affect the oscillation frequency.
Harmonic oscillators on the other hand generate a sinusoidal signal. FIG. 2 shows such a circuit. The output of an amplifier 201 is fed to the input of the amplifier through a filter 202 as shown in FIG. 2. The phase shifter 203 ensures that the oscillations are sustained by adjusting the overall loop phase to zero. The quality factor of the filter limits the stability of the frequency and the phase noise of the oscillator. For increased accuracy, mechanical elements such as quartz crystals or MEMS structures with high Q are used as the frequency determining elements.
In many integrated MEMS devices, the clock signal is usually generated by a relaxation oscillator due to its simplicity, small area requirement and low power consumption. However, the frequency of this circuit is a function of the resistive and capacitive circuit components which may change with process parameters or temperature. For example, typically the resistor values change 10% over 100 degree C., resulting 10% frequency shift over the specified temperature range. On the other hand, a crystal filter provides a very accurate clock signal but using these filters is prohibited by the cost and size requirements of the consumer applications.
There is a need for generating an accurate timing base in MEMS devices without using an external quartz crystal or another source. The present invention addresses such a need.